Glow discharge apparatus for treating workpieces



March 7, 1967 c JONES ET AL 3,308,049

GLOW DISCHARGE APPARATUS FOR TREATING WORKPIECES Filed March 6, 1963 2 Sheets-Sheet l FIGI.

[/7 ven fo rs C/auoe /f. Jones Siuari l V/Marf/n The/r Afforneg March 7, 1967 c JONES ET AL GLOW DISCHARGE APPARATUS FOR TREATING WORKPIECES 2 Sheets-Sheet 2 Filed March 6, 1963 FIG.4.

Sm e f wm ro o oJM W AW A u mm m S m United States Patent Office 3,398,949 Patented Mar. 7, 1967 3,308,049 GLOW DISCHARGE APPARATUS FOR TREATING WORKPIECES Claude K. Jones, Marblehead, and Stuart W. Martin,

Salem, Mass, assignors to General Electric Company, a

corporation of New York Filed Mar. 6, 1963, Ser. No. 264,167 1 Claim. (Cl. 204-312) This invention relates to glow discharge apparatus, wherein a work piece disposed in a partially evacuated housing is subjected to various processes through ionic bombardment of the Work piece by suitable gases contained in the housing, the current passing between housing and work piece being in the form of a glow discharge surrounding the work piece. More particularly, the invention relates to an improved circuit for controlling a cathode glow discharge so as to prevent arcing, or local overheating of the work piece.

Proper control of glow discharge apparatus at high currents or at elevated temperatures, so as to avoid deterioration of the uniform glow into local arcing, is quite difficult. Local overheating or other temporary irregularities can cause the glow discharge to break down and result in an unstable condition leading to arcing, which has a damaging effect on the work piece.

The difiiculty of controlling the glow discharge so as to prevent such arcing has led to suggestions for incorporating a relatively high resistance in series with the electrodes of the glow discharge apparatus, or, in some cases, to intermittently apply a resistance in series with the glow discharge apparatus in accordance with some controlling parameter such as measured temperature of the work piece. A load line is thus established which results in a substantial drop in voltage applied to the electrodes, when the current increases. In such cases, a substantial amount of power is consumed in the series resistance, which power might otherwise be used in conversion of the work piece through the desired glow discharge process.

Other sugaestions have included discontinuous applications voltage from the power source, such as through the use of pulsating D.-C. current, by which means the local overheating effects are averaged out to reduce the tendency toward arcing. Again, full utilization of the power source is not had, where there are periods during which no energy is transferred.

Accordingly one object of the present invention is to provide an improved glow discharge control circuit which reduces tendency toward unstable arcs.

Another object of the invention is to provide an improved glow discharge control circuit wherein full utilization is made of the power source with continuous transfer of energy to the work piece.

Still another object of the invention is to provide an improved glow discharge control circuit, wherein the glow discharge current is maintained relatively constant without excessive power losses in the current controlling device.

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIG. 1 is a schematic view of a glow discharge apparatus, employing a glow discharge control circuit according to the preferred form of the invention,

FIG. 2 is a graph of a typical glow discharge,

FIG. 3 is a graph illustrating the operation of the embodiment shown in FIG. 1,

FIG. 4 is a simplified schematic view of a modified form of the invention,

FIG. 5 is a graph illustrating the operation of the FIG. 4 modification,

FIG. 6 is a simplified view of still another modified form of the invention,

FIG. 7 is a graph illustrating the operation of the FIG. 6 modification, and

FIG. 8 is a partial circuit illustrating a variation of the modification of FIG. 6.

Briefly stated, the invention is practiced by connecting in series with the glow discharge electrodes a control device which has a high efiective impedance to small changes in glow discharge current at the operating point, but which has a relatively low D.-C. voltage drop and hence consumes relatively small amounts of power.

Referring now to FIG. 1 of the drawing, a simplified schematic view of a glow discharge apparatus shows it to include a metal housing 1, hermetically sealed to a base 2, to form a chamber 3. An electrode 4, extends into chamber 3 and is insulated from base 2 by means of a gas-tight insulator 5. Means for evacuating chamber 3, include a pipe 6, connected by valves 7, 8, to a vacuum pump 9. Means for supplying small quantities of gas to chamber 3 for use in the glow discharge process include a gas bottle 10, connected by valve 11, to pipe 6.

A work piece 12, is electrically connected to electrode 4, and may also be supported thereby. The cathode glow enveloping work piece 12, is indicated by dotted lines 13. Suitable means for monitoring the temperature of work piece 12, include a temperature indicator 14, which may be a suitable radiation pyrometer or an equivalent thermocouple device.

A source of D.-C. electric potential 15, is shown here as a battery for simplicity, but ordinarily it would be a regulated source of D.-C. voltage such as a full-wave rectifier, with suitable filters, supplied by an AC. source. The positive terminal of D.-C. voltage source 15 is connected to the metal housing 1 by means of a lead 16, and electrode connection 17. For smaller work pieces, the electrode 17, would extend into housing 1 through a gastight insulator similar to insulator 5 to be disposed a short distance from the work piece, but here the entire housing 1 and base 2, acts as an anode. For safety reasons, therefore, a ground connection 18, is included to hold housing 1 at ground potential.

The gas used in bottle 10, and the pressure maintained within chamber 3, depends upon the type of process to be carried out. For example, nitriding of work piece 12 can be accomplished if the gas supplied from bottle 10 is a mixture containing nitrogen and the pressure in chamber 3 is held at a few mm. Hg.

The negative terminal of D.-C. power source 15 is connected by Way of lead 19 to the cathode 20a of a vacuum tube pentode 20. Only one pentode 20 is shown, although it will be understood that several pentodes would be connected in parallel in practice according to the desired current requirements. A feedback circuit (not shown) might also be included to increase the effective impedance. The plate 2% of the pentode is connected to electrode 4 by means of a lead 21. Thus, a DC. series circuit is established from D.-C. power source 15 through lead 16, electrode 17, the ionization path of glow 13, work piece 12, electrode 4, lead 21, pentode 20, and lead 19. As will be explained, the pentode serves as a current limiting means to stabilize current flow in the flow discharge 13.

Suitable bias is established on the screen grid 200 of pentode 20 by means of D.-C. voltage source 22 and adjustable voltage divider 23. Current flow across the pentode between cathode 29a and plate Zilb is controlled by means of a second D.-C. voltage source 24 and adjustable voltage divider 25. Movement of arm on voltage divider 25, changes the negative bias on the control grid d and thus determines the current flow in the series circuit.

Reference to FIG. 2 of the drawing indicates the characteristic curve of a typical glow discharge which might take place inside chamber 3. The current I flowing between electrodes 4 and 17, is indicated along the horizontal axis while the potential V between electrodes 4 and 17, is indicated on the vertical axis. It will be noted that the normal glow region, in which it is desired that the conversion process be maintained, is indicated along a fiat portion 27, on the curve. Portion 2% of the curve is an abnormal glow region, which is unstable and will rapidly change into the arc region designated as portion 29 of the curve. It is to be noted that portion 29 is characterized by rapidly decreasing potential difference across electrodes 4 and 17, together with an increased current flow.

Previous arrangements for prevention of the glow dis charge proceeding into the arcing region have included a series resistance in place of, or intermittently connected in place of, pentode 29. Such a prior art arrangement would establish a load line such as indicated by the dashed line 30, which establishes an operating point at its intersection with the giow discharge curve. Any increase in glow current, therefore, results in decreased voltage applied between electrodes 4 and 17, thus holding current steady. The objection to controlling the current flow through the glow discharge apparatus by means of a series resistor, as described, is that a relatively large resistance must be used. This means that a substantial amount of power is lost in merely heating the resistor.

In accordance with the invention, pentode Ztl is em ployed as a current-limiting means. Pentode 20 has a high effective impedance to small changes in glow current, thus resisting tendency of the glow discharge apparatus toward arcing. The pentode, however, actually consumes relatively little power due to the relatively small D.-C. voltage drop thereacross.

FIG. 3 illustrates the characteristic curves of a typical pentode. Adjustment of the control grid voltage through the movable arm 26 will serve to select a curve, such as 31, which defines the operating characteristics of pentode 20 for changing potentials E between its plate and cathode. It will be observed that since curve 31 is extremely non-linear, and since it has a fiat portion, wherein there is relatively little change in current flow with changes in applied potential, there is a high effective impedance across pentode 20 to small changes in applied potential along this flat portion of the curve. On the other hand, the D.-C. drop across pentode 20, which is a true measure of the power consumed therein, will be relatively modest. For example, the effective impedance presented by pentode 21} for small changes (equivalent to dynamic plate resistance) might be measured in megohms, whereas the D.-C. voltage drop (wherein power is actually consumed) might be only a few thousand ohms. A suitable pentode would be type GL829B. A typical application would employ 8 such tubes connected in parallel.

The operation of FIG. 1 may be understood by reference to FIG. 3, wherein a load line 32, considering the glow discharge apparatus as the load device, intersects curve 31 at an operating point P. The tendency of the glow to deteriorate into an arc,,as indicated on portion 29 of the curve in FIG. 2, represents a decreased resistance across the glow discharge space. This manifests itself as a tendency to form a new load line 33, in FIG. 3, with a new operating point P. It will be observed that along the flat portion of curve 31, there is very little current change through the pentode 20 for relatively large changes in applied potential E. Therefore, any current change is limited by a high effective impedance, which resists tendency of the glow to deteriorate into an arc.

Explained in another way, a reduced voltage drop between electrodes 4 and 17 results in an increased voltage drop across pentode 25*, but due to the fiat voltagecurrent characteristic of the pentode at the operating point, very little change in current flow can take place. It should be noted again that, although there is an apparent high impedance connected in series with the glow electrodes, the D.-C. voltage drop across the current limiting device is relatively small and consumes little power.

Reference to the modification shown in FIG. 4-, in which it will be understood that the same or equivalent accessory devices as in FIG. 1 would be employed but are omitted for clarity, a housing 40 contains a work piece il enveloped in a cathode glow discharge iii. A DC. voltage source 43 has its positive terminal connected to housing 44 which again serves as the anode and its negative terminal connected to the cathode 44a of a thermionic diode 14. The plate idb of the diode is connected to work piece 41, forming a series circuit as before. Temperature of the cathode 44a of the diode is controlled by a heater filament 44c, energized by means of a battery 45 and an adjustable rheostat 46.

Reference to FIG. 5 shows the characteristic curve of such thermionic diode again to be non-linear, consisting of a rapidly rising portion 47 and a flat portion 48, wherein the diode operates in a saturated condition, dependent only on the temperature of cathode 44a. Diode id is selected so that the current flow at an operating point, indicated as 49 on the flat portion 48 of the curve, is suitable for the glow discharge taking place in housing 4! A suitable diode would be type GU4G. As before, several of such tubes would generally be employed connected in parallel.

In operation, the thermionic diode, along the saturation portion 48 of its curve, operates similarly to the aforedescribed pentode 20, in that large changes in applied potential E across diode 44 produce very little change in current flow through the glow discharge apparatus. On the other hand, since the characteristic curve of diode 44 is not linear, the D.-C. voltage drop across diode 44, which is indicative of the power consumed therein and diverted from the glow, is relatively small.

FIGURE 6 illustrates still another modification of the invention, wherein housing 50 contains work piece 51 as before, which is enveloped in a glow discharge indicated by dotted lines 52. A D.-C. voltage source 53 supplies a potential between housing 5t] and work piece 51, in a series circuit which includes a rectifier 54. Rectifier 5 is solid state with a pn junction fabricated by conventional techniques. It is to be particularly noted that rectifier 54 is connected with its polarity reversed from that allowing forward flow of current through the rectifier. In other words, current flows from the positive terminal of voltage source 53, across the ionized space be tween the glow discharge electrodes, and in the reverse direction through rectifier 54, back to source 53. Several rectifiers 54 may be employed connected in parallel.

Reference to FIG. 7 illustrates the characteristic curve of a rectifier such as shown at 54. Forward current fiow is indicated by the portion of the curve designated 55, whereas reverse current flow is indicated by the relatively fiat portion 56, followed by portion 57 indicating diode breakdown. The current flow permitted in the reverse direction (portion 55) will be very substantially less than that in the forward direction (portion 55), but by suitable selection of rectifier 54, a reverse current capability can be selected, which is also suitable for a glow discharge. Suitable operation would be indicated at a point such as 53 on the fiat part of the characteristic curve in FIG. 7.

The operatiorrof the modification of FIG. 6 is similar to that of the aforementioned arrangements, wherein a current limiting device having a non-linear voltage-current characteristic is employed at an operating point selected on the flat portion of the voltage-current characteristic. Thus, the elfective impedance which resists the tendency to are is very high, whereas actually the D.-C. voltage drop across the current limiting device for a given current level is relatively low.

A variation of the modification of FIG. 6 may be seen in the partial circuit of FIG. 8. Here a silicon-controlled rectifier 59 such as type 2N1595 is employed instead of the ordinary solid state rectifier 54 of FIG. 6. It is connected for reverse current flow as before. Additional flexibility in setting the reverse current level is provided by means of a variable resistor 60 and a source of potential 61 forming a trigger circuit 62, The trigger circuit is not employed in the usual way to gate a forward voltage, biifinstead is used to set'the'reverse current level by adjusting the injection current from source 61 with resistor 60. In this respect, its operating characteristics in the reverse current mode would appear much as shown in FIG. 7. Several such silicon-controlled rectifiers may be used in parallel as before.

Thus it will be seen that the various modifications of the control circuit shown effectively control the glow discharge apparatus, resisting tendency for it to depart from the normal glow region and to deteriorate into a harmful arcing condition. This control is achieved without excessive power consumption and at a constant level of energy supplied to the glow.

While there has been described herein what are considered to be preferred embodiments of the invention, still other modifications will occur to those skilled in the art, and it is desired to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters 6 Patent of the United States is:

The combination of:

a sealed housing adapted for placement of a workpiece therein,

means for evacuating said housing and for introducing small quantities of ionizable gas,

spaced electrode means arranged for establishing a glow discharge within the housing, at least one of said electrode means being arranged to be connected to the workpiece,

a source of constant D.-C. electric potential,

at least one pentode tube having a plate and cathode connected in series with said electrode means across said potential source, and

adjustable biasing means connected to select an operating point for said pentode at a D.-C. resistance suitable for maintaining a constant glow between said spaced electrodes, said operating point also lying on a relatively constant-current portion of the pentode characteristic curve.

References Cited by the Examiner UNITED STATES PATENTS 1,662,114 3/1928 Hotchner 3l5--20l 2,146,818 2/1939 Hahnle 315-205 2,700,110 1/1955 Shamos 25083.6 2,852,721 9/1958 Harders et a1. 3l5-205 X 2,897,429 7/1959 Jochems 307-88.5l5.2 2,911,571 11/1959 Knuppel 315--243 X 3,202,904 8/1965 Madland 3l5--209 HOWARD S. WILLIAMS, Primary Examiner.

JOHN H. MACK, Examiner.

G. KAPLAN, Assistant Examiner. 

